Method and apparatus for cooling a cryogenic storage container

ABSTRACT

A method of conserving the refrigeration value of a dewar by increasing the thermal conductivity of a selected portion of an otherwise low thermal conductivity conduit and restricting the effective cross sectional area along at least as much as the conduit as the higher conductivity portion so as to increase the velocity of the boil-off gases and permit more of the refrigeration value of the boil-off gas to pass through the conduit&#39;&#39;s high conductivity portion from which the refrigeration is directed into the dewar&#39;&#39;s vacuum space. A preferred structure for performing the above method includes a high thermal conductivity collar about the conduit and a high conductivity shield extending from the collar into and through the dewar&#39;&#39;s vacuum space. A second tube is placed within the conduit to form an annular space therebetween. One end of the second tube is closed so that the boil-off gases are forced through the annular space which is filled with a fine fibrous relatively non-conductive packing material in an area that extends for a distance at least as long as the collar. In this manner, the boil-off gas velocity is increased as it passes through the packing material and results in an increase in the transfer of the boil-off gas&#39;&#39; refrigeration value to the collar, shield, and vacuum space.

United States Patent DeHaan [54] METHOD AND APPARATUS FOR COOLING A CRYOGENIC STORAGE CONTAINER [72] Inventor: James R. DeHaan, Boulder, C010.

[73] Assignee: Cryogenic Engineering Company,

Denver, C010.

22 Filed: N6v.3,1969' [21 Appl. No.: 873,313

52] 05.01. ..62/54,165/l79, 165/181, 1 220/9, 220/14 51 Int. Cl. ..F17c 13/00 '[58] Field of Search ...62/45, 54; 220/9, 14; 165/179,

[56] References Cited UNITED STATES PATENTS 3,133,422 5/1964 Paivanas et al. ..62/54 x 3,265,236 8/1966 Gibbon et al 3,538,714 11/1970 Klipping et al. ..62/54 FOREIGN PATENTS OR APPLICATIONS A 984,095 2/1965 GreatBritain "L; ..220/l4 1,525,259 4/1968 France ..62/54 Primary Examiner-Albert W. Davis, Jr. Attorney-Griffin, Branigan and Kindness 1451 Dec. 12,1972

} [57] ABSTRACT A method of conserving the refrigeration value of a dewar by increasing'the thermalconductivity of 21 Selected portion of an otherwise low thermal conductivity conduit and restricting the effective cross sectional area along at least as much as the conduit as the higher conductivity portion so as to increase the velocity of the boil-off gases and permit more of the refrigeration value of the boil-off gas to pass through the conduits high conductivity portion from which the refrigeration is directed into the dewars vacuum space.

A A preferred structure for performing the above method includes a high thermal conductivity collar about the conduit and a high conductivity shield extending from the collar into and through the dewars vacuum space. A second tube is placed within the conduit to form an annular space therebetween. One end of the second tube is closed so that the boil-off gases are forced through the annular space which is .filled with a fine fibrous relatively non-conductive packing material in an area that extends for a distance at least as long as the collar. In this manner, the boiloff gas velocity is increased as it passes through the packing material and results in an increase in the transfer of the boil-off gas refrigeration value to the collar; shield, and vacuum space.

32 Claims, 3 Drawing Figures PATENTED HEB 12 I972 FIG. 2

INVENTOB JAMES R. DE HAAN BY Qm m, Bra/WM a Kmm.

AT'IOHN MYS 1 METHOD AND APPARATUS FOR COOLING A CRYOGENIC srORAeE CONTAINER Many systems have been devised for limiting the heat that is permitted to leak into cryogenic storage vessels. Several such systems relate to structures for recovering some'of the refrigeration value from the gases which boil out of the vessels interior. Devices of this type are described in U.S. Pat. Nos. 3,133,422 and 3,341,052. The structures described therein include a plurality of highly conductive shields in the vessels vacuum space. The shields are affixed to a heat absorbing fluid conduit so that refrigeration from boil-off gases passing through the conduit is first given up to the conduit and then passed on to the shields so as to cool the vacuum space. It is an object of this invention to provide a method and apparatus for further increasing the amount of boil-off A gas refrigeration that is transmitted to the vessels vacuum space.

. A drawback of previous devices is that relatively large numbers'of shields have been required, thereby resulting in structural complexity and expense, particularly in the case where fins have been included in the interior portions of th'econduit. Consequently, another object of this invention'is to provide a method and apparatus for maintaining a high rate of refrigeration transfer from the vessels boil-off gas to the vessels vacuum space while at the same time reducing the number of shields that are required.

The use of fins on the interior portions of the vessels conduits has been limited to use by either relatively large "conduits or has required impractical and complex manufacturing procedures. Hence, it is another object of this invention to provide a structure which has a very high rate of transfer of boil-off gas refrigeration to the vessel's vacuum space even though the particular conduit'may be of small diameter.

In accordance with the principles of the invention, the vessels fill and vent conduits are comprised of a lowthermal conductivity material. A selected portion of the conduit intermediate its ends is surrounded by a high thermal conductivity .collar which functions to provide a highly conductive zone of the surrounded tube. The collar should be at least one inch long so as to make at least that much of the conduit highly thermally conductive. In addition, the velocity of the boil-off gases is increased in the portion of the conduit surrounded by the collar by means of a packing material. It has been found that this increase in velocity also increases the refrigeration that is transferred from the conduits boil-off gas to the collar. It has also been found that results are still further increased if the packing is comprised of a fine fibrous low conductivity material such as terepthalic ester polymer fibers that are packed in the conduit for at least as much of its length as is surrounded by the collar. A shield which extendsthrough the vessel's vacuum space is then attached to the collar so that the thusly transferred refrigeration is distributed through the vessels vacuum space. In this respect, in order for the refrigeration to travel to the shields, the highly conductive collar should surround the conduit at an area that is colder than the shield would be if not connected to the collar.

The foregoing and other objects, features, and advantages of the invention will be apparent from the more particular description of preferred embodiments thereof as illustrated in the accompanying drawings, wherein the same reference numerals refer tov the same parts throughout the various views. The drawings are not necessarily intended to be to scale, but rather are presented so as to illustrate the principles of the invention in clear form.

In the drawings:

FIG. 1 is a pictorial view of a dewar vessel of the type in which this invention finds particular utility;

FIG. 2 is a fragmentary sectional view taken along the arc 2--2 in FIGQl;

FIG. 3 is an alternative embodiment of the invention which is particularly suited for use with relatively small conduits such as vent lines. I

In FIG. 1 a conventional dewar vessel 10 has a neck portion 12 having an outlet tube 14 extending therefrom. The neck 12 is equipped with a flanged cap I 16 having a relief valve 18 therein. This, structure has been cut-away'in FIG. 2 which illustrates the vessel 10 as having an outer shell 20 enclosing an inner container 22 for a'cryogenic fluid, not shown. The space between the two walls 20 and 22 is evacuated to form an insulating vacuum space 24.

The vacuum jacket preferably contains one or another of a variety of bulk insulation materials. One of the more satisfactory types of bulk insulations is comprised of a plurality of radiation barriers 26 which are separated from each other by a suitable low conductive fibrous material 28. For purposes of simplicity, only a small portion Of that type of insulation is illustrated in the drawings, but in practice the vacuum space 24 is usually substantially completely filled with such a laminated insulation. In this respect, a multi-layer insulation of this type is described in more detail in an article by Dr. Richard H. Kropschot of the National Bureau of Standards. This article appears in the March 1961 issue of Cryogenics, Vol. I, No. 3, and is entitled Multiple Layered Insulation for Cryogenic Application. A suitably opacified powder type of insulation can also be employed in the vacuum space. These matters, however, form no part of the instant invention and will not be further discussed.

The inner container 22 is supported from the outer V shell 20 by means of a low thermally conductive neck tube or conduit 30 which is preferably made of a fiberglass reinforced epoxy resin or stainless steel and should have as thin a wall as possible, compatible with the strength required to support the inner container 22 and its contents. The neck tube 30 is surrounded by a collar 32 of a highly thermally conductive material such as aluminum, and is suitably affixed to the exterior of the neck 30 so as to insure good thermal contact therebetween. The collar 32 functions to increase the thermal conductivity of the portions of the neck tube 30 which it surrounds. In this respect, it has been found that the height of the collar (between lower surface 34 and upper surface 36) should be a minimum of about 1 inch in order to provide a suitably wide area of the neck tube 30 having an increased thermal conductivity. Collars as high as four inches have also been found satisfactory, but it should be noted that the temperature differential between surfaces 34 and 36 of the collar is only a few degrees. Hence, the effective thermal length of the neck tube 30 is reduced. The overall length of the neck tube 30 should be proportionately increased, therefore, to maintain a sufficiently small transfer of heat from the vessel's surroundings, down the neck tube 30 to the inner container 22.

A highly thermally conductive shield 38 is affixed to the collar 32 so as to surround the inner container 22 within the vacuum space 24; and a second low thermal conductivity tube 40 is located within the neck tube 30 to form an annular space 42 between the two tubes. The top of the inner tube 40 is covered by a cap 44 having a pressure relief member mounted therein. Hence, boil-off gases from the container 22 are forced to travel upwardly through the annular space 42 and out a vent hole 48 in the neck tube 30 so as to be exhausted through the outlet tube 14.

In the absence of its connection to the collar 32 the shield 38 would be at a given temperature depending upon its location between the outer and inner vessel walls 20 and 22. Consequently, in order for the boil-off gas refrigeration to be passed on to the shield, the collar 32 should surround an area of the outer conduit 30 that is at a temperature lower than that of an unconnected shield similar to 38. a

A fine fibrous low conductive packing material 50 is comprised of terephthalic ester polymers, one type of which is identified by the trademark DACRON. Such fibers having diameters of greater than about microns, but less than about 100 microns are placed in the annular space 42 adjacent the collar 32. In this respect, packing materials have also included metal wool, highly conductive screening, and a narrow metalic ribbon, but as will be described shortly, better results are obtained when the relatively non-conductive fibrous packing material is used. A packing material of either type functions to restrict the cross sectional area of the annular space 42 and increase the velocity of the boil-off gas as it passes the area of the collar 32. In addition, the fine fibrous fibers dampen certain heat producing vibrations which tend to occur otherwise. The packing material also causes an increase in the turbulence of the boil-off gas and in addition, in the case of the metallic packing, provides a path for the transfer of cold from the inner tube 40 to the outer tube 30. Whichever type of packing is used, the increased velocity of the boil-off gas causes an increase in the refrigeration that is transferred to the collar 32, the shield 38 and the vacuum space 24. Packing plugs have certainly been used in the past to restrict gas flow. Frequently, however, such plugs have caused problems because they have encouraged the formation of frozen air in the vessel's neck tube. The instant structure avoids the undesirable effects of this problem because in the event of such freezing the vessels boil-off gases can still exit through the inner conduit 40 and the pressure relief valve 18.

The above described structure also provides substantial increase in the surface area to which the refrigeration value of the boil-off gas can be transferred. That is, the boil-off gases pass over all of the surfaces of the packing material as well as the inner and outer tube surfaces adjacent the packing material. Hence, the refrigeration value of the boil-off gas is transferred to the collar 32 by both forced gas convection as the gas is forced through the annular space at a high velocity; and at least where metallic packing is employed by solid conduction from the inner tube 40, through the metallic packing material and through the portion of the neck 30 that is surrounded by the collar 32. In these respects, it should be noted that best results are obtained when the packing material extends along at least as much of the length of the annular space as is surrounded by the collar 32.

On occasion, the boil-off gas flows out of a dewar vessel at an excessively high rate. This occurs, for example, during filling or perhaps when this vessel is damaged and its vacuum is lost. For this additional reason, therefore, it has been found desirable to include the pressure relief valve in communication with the central tube 40.

It has been found that the above described structure results in both a minimum tendency for heat to flow downwardly into the interior portions of the storage vessel while still permitting the transfer of a considerable amount of the boil-off gas refrigeration to be transferred to the vessels vacuum space. Moreover, this is accomplished by the relatively simple structure of a collar 32 surrounding the low thermal conductivity neck tube 30 and the annular portion between the inner and outer tubes having an increased flow velocity by means of a packing extending along at least as much of the annular space as is surrounded by the'collar. This structure, therefore, eliminates the need for the large plurality of heat transfer shields that have previously been used. Hence, the structure of the invention reduces both the cost and complexity of the dewar's fabrication, and at the same time provides an increase in the vessels overall insulation efficiency. Moreover, the vessels efficiency is still further increased where a fine fibrous low conductive packing is employed.

The annular space 42 between the inner and outer tubes 40 and 30 is preferably about one-sixteenth to one-eighth inch wide and the height of the collar is preferably between about 1 to 4 inches for dewars having capacities of between 50 to 500 liters. The height of the collar 32 can be varied from this height range, but the effectiveness thereof is rapidly decreased when it is less than one inch in height; and a height of more than about four inches would sometimes result in an inconveniently long neck tube.

As noted above, in one preferred embodiment of the invention, the packing material was comprised of fibers of terephthalic ester polymers of between about 10 and microns in diameter. This fine fibrous packing was placed in a low thermal conductivity conduit of a 50 liter dewar in the manner described above and the dewar was subjected to heat transfer tests to determine its boil off rate. In this respect, the vessels loss rate was only 1.2 percent per day which is a substantial improvement over the heat loss that is expected for corresponding vessels that lack the improvements of this invention. In fact, it is even a substantial improvement over an otherwise similar test dewar which used only a )6 inch copper screen packing within a 1 inch collar. That copper screen dewar" had an indicated heat loss of 1.6 percent per day which itself is quite an improvement over corresponding but otherwise conventional dewars. But, the fine fiber packing embodiment represents a 25 percent improvement over even the improved results obtained by the copper screen dewar Hence, it will be appreciated by those skilled in the art that the invention provides admirable results even though they are obtained in a manner which is in many respects less complicated then the prior art.

it has also been found that additional shield and collar structures can be employed satisfactorily. In this event, however, the collar 32 corresponding to the colder of the two shields should be made shorter than the collar for the warmer shield. But in any case, for a given amount of boil-off gas refrigeration that is transferred into the vessels vacuum space, the required number of shields 38 does not approach the number that have been required in the prior art devices which have not employed elements corresponding to collar 32 in combination with related restricted-flow portions such as that provided by the packing material 50 placed in the annular space 42 of the instant structure. Thus far, the invention has been described in terms of a relativelylarge neck tube which is used to support thevessels inner storagecontainer. Frequently, however, small vent tubes extend fromthe vessels inner container, and through the vacuum space to the outer wall. Although it might not-be practical to install a separate tube inside of such a vent line, it is nevertheless practical to practice the inventions broader concepts. For example, w ith refe'rence to FIG. 3, a vent line 60 extends from the vellels inner wall 22 to its outer wall 20. In this respect, the vent tube 60 is illustrated as being straight, but conventionally, such tubes are bent several times for any number of reasons prior to the time that they emerge from the vessels outer wall. The vent tube 60 is surrounded by a collar 62 corresponding to a collar 32 in the neck-tube embodiment of the invention. The collar62 has a lower surface 64, an upper surface 66, and is connected to a shield 68 which extends throughout the vessel s vacuum space in the same manner as was described above.

The vent tube 60 is filled with a terephthalic ester polymer fiber packing material 70 in the cross section of the tube extending at least along the length bounded by the lower and upper surfaces 64 and 66 of the collar 62. In this manner, the packing material 70 functions to most efficiently. increase both the velocity and turbulence of the boil-off gases through tube 60.

Both the increased velocity and the increased turbulence are effective to increase the rate of transfer of the refrigeration from the boil-off gas to the portion of the wall 60 that is surrounded by the collar 62. Hence, the amount of refrigeration that is permitted to pass from the collar 62 to the shield 68 is increased considerably over the amount of refrigeration that would be transferred in the absence of either the collar or the packing. Moreover, as in the case of the first embodiment, the above described structure eliminates the need for the previously required large number of shields while at the same time providing a low thermally conductive vent tube material so as to decrease the amount of heat flowing into the vessel through the walls of the vent tube 60 from the surrounding air to the inner container 22.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it should be noted that various changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, although the invention has been described in connection with a standard neck-type of dewar, it will be ap 'preciated by those skilled in the art that the invention is equally applicable to the larger horizontal types of dewars, such as those that are sometimes mounted on mobile platforms. Similarly, the conduit can have the highly thermally conductive portion formed as a part thereof rather than as a collar; and part of the restricted portion of the conduit can be obtained by reducing its diameter at a selected section. Such modifications will be contemplated by those skilled in the art, however, and will not be described in more detail.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: I

1. In a double walled cryogenic storage vessel having a cold storage container wall separated from a warm outer wall by a vacuum space in which at least one radiation shield is located, and being of the type in which a low thermal conductivity conduit of a given cross sectional area extends from said container wall through said vacuum space to said outer wall, the method of conserving'the refrigeration value of the boil-off gases from said container comprising the steps of:

increasing the effective thermal conductivity between the low thermal conductivity conduit and the radiation shield for at least about one inch along the length of said conduit at a selected portion intermediate the ends thereof;

directing said boil-off gases through said conduit;

and,

restricting the effective cross sectional area of said conduit at said selected portion along at least the same length as that for which the thermal conductivity is increased to increase the velocity of said gas as it flows through the restricted portion of said conduit to thereby increase the transfer of cold from said boil-off gas to said selected portion of said conduit.

2. The method of claim 1 including the step of increasing the turbulence of said boil-off gas as it flows through the restricted portion of said conduit.

3. The method of claim 2 wherein said turbulence is increased by directing said boil-off gas through a fibrous packing material.

4. The method of claim 1 including the step of increasing the turbulence of said boil-off gas as it flows through said restricted portion of said conduit.

5. In a double walled cryogenic storage vessel having a cold storage container wall separated from a warm outer wall by a vacuum space, the combination comprising:

a low thermal conductivity conduit extending from said container wall through said vacuum space to said outer wall for carrying cold boil-off gas out of said storage container; collar of highly thermally conductive material located in said vacuum space and surrounding a selected portion of the outer surface of said conduit so as to be in heat transfer relationship with said outer surface for at least about one inch along the length thereof;

a highly thermally conductive shield affixed to said collar and extending into said vacuum space; and,

restriction means in said conduit for restricting the cross sectional area of said conduit along at least the same length as that for which said collar extends, said restriction means being operative to increase the velocity of said boil-off gas as it passes through said selected portion.

6. The apparatus according to claim '5 wherein the restriction means is comprised of a packing material in said conduit in heat transfer relationship with the portion of the inner surface of said conduit adjacent said selected portion of said outer surface.

7. The apparatus of claim 6 wherein said restriction means has an effective thermal conductivity of about that of terephthalic ester polymer fibers.

8. The apparatus of claim 6 wherein said packing material is fibrous and comprised of fibers having diameters of less than about 100 microns.

9. The apparatus of claim 8 wherein said fibers are comprised of terephthalic polymer fibers.

10. The apparatus according to claim including a second low thermal conductivity conduit located inside of the first conduit to form a channel for the flow of said boil-off gas between the inner wall of said first conduit and the outer wall of said second conduit, said restriction means being located in said channel; and,

means for normally preventing the flow of boil-off gas through sa id second conduit.

11. The apparatus .of claim 10 wherein said means for normally preventing the flow of boil-off gas through said second conduit is responsive to more than a predetermined amount of pressure in said storage container to permit said boil-off gas to pass through said second conduit so as to relieve said pressure.

12. The apparatus of claim 10 wherein the restriction is comprised of a packing material in heat transfer relationship with the outside surface of said second conduit and the inside surface of said first conduit.

13. The apparatus of claim 10 wherein said restriction means has an effective thermal conductivity of about that of terephthalic ester polymer fibers.

14. The apparatus of claim 10 wherein said packing material is fibrous and comprised of fibers having diameters of less than about 100 microns.

15. The apparatus of claim 14 wherein said fibers are comprised of terephthalic ester polymer fibers.

16. The apparatus of claim 10 wherein said second conduit is spaced from said first conduit by no more than about one-eighth inch.

17. In a double walled cryogenic storage vessel having a cold storage container wall separated from a warm outer wall by a vacuum space, the combination comprising: 1

a low thermal conductivity conduit extending from said container wall through said vacuum space to said outer wall for carrying cold boil-off gas out of said storage container;

a collar of highly thermally conductive material located in said vacuum space and surrounding a selected portion of the outer surface of said conduit so as to be in heat transfer relationship with said outer surface for at least about one inch along the length thereof;

a highly thermally conductive shield affixed to said collar and extending into said vacuum space; and, a fibrous packing material in said conduit in heat transfer relationship with the portion of the inner surface of said conduit adjacent said selected portion of said outer surface. 18. The apparatus of claim 17 wherein said packing material has an effective thermal conductivity of about that of terephthalic ester polymer fibers.

19. The apparatus of claim 18 wherein said fibers have a diameter of less than about microns.

20. The apparatus of claim 17 wherein said packing material is comprised of terephthalic ester polymer fibers having diameters of less than about 100 microns.

21. The apparatus of claim 17 including a second low thermal conductivity conduit located inside of the first conduit to form a channel for the flow of said boil-off gas between the inner wall of said first conduit and the outer wall of said second conduit, said fibrous packing material being located in said channel; and,

means for normally preventing the flow of boil-off gas through said second conduit.

22. The apparatus of claim 21 wherein said second conduit is spaced from said first conduit by no more than about one-eighth inch.

23. The apparatus of claim 21 wherein said means for normally preventing the flow of boil-off gas through said second conduit is responsive to more than a predetermined amount of pressure in said storage container to permit said boil-off gas to pass through said second conduit so as to relieve said pressure.

24. The apparatus of claim 21 wherein said packing material has an effective thermal conductivity of about that of terephthalic ester polymer fibers.

25. The apparatus of claim 24 wherein said fibers have diameter of less than about l00 microns.

26. The apparatus of claim 21 wherein said packing material is comprised of terephthalic ester polymer fibers having diameters of less than about 100 microns.

27. The apparatus of claim 17 including at least a second highly thermally conductive collar located in said vacuum space and surrounding at least a second selected portion of the outer surface of said conduit so as to be in heat transfer relationship with said outer surface, the height of the collar closer to said cold storage container wall being less than the height of a collar being further from said storage container wall.

28. The apparatus of claim 27 including a highly thermally conductive shield affixed to each of said collars, each of said shields extending into said vacuum space.

29. The apparatus of claim 27 wherein said collar closer to said cold storage container is at least about one inch high.

30. The apparatus of claim 27 wherein said packing material has an effective thermal conductivity of about that of terephthalic ester polymer fibers.

31. The apparatus of claim 30 wherein said fibers have a diameter of less than about 100 microns.

32. The apparatus of claim 27 wherein said packing material is comprised of terephthalic ester polymer fibers having diameters of less than about 100 microns. 

1. In a double walled cryogenic storage vessel having a cold storage container wall separated from a warm outer wall by a vacuum space in which at lEast one radiation shield is located, and being of the type in which a low thermal conductivity conduit of a given cross sectional area extends from said container wall through said vacuum space to said outer wall, the method of conserving the refrigeration value of the boil-off gases from said container comprising the steps of: increasing the effective thermal conductivity between the low thermal conductivity conduit and the radiation shield for at least about one inch along the length of said conduit at a selected portion intermediate the ends thereof; directing said boil-off gases through said conduit; and, restricting the effective cross sectional area of said conduit at said selected portion along at least the same length as that for which the thermal conductivity is increased to increase the velocity of said gas as it flows through the restricted portion of said conduit to thereby increase the transfer of cold from said boil-off gas to said selected portion of said conduit.
 2. The method of claim 1 including the step of increasing the turbulence of said boil-off gas as it flows through the restricted portion of said conduit.
 3. The method of claim 2 wherein said turbulence is increased by directing said boil-off gas through a fibrous packing material.
 4. The method of claim 1 including the step of increasing the turbulence of said boil-off gas as it flows through said restricted portion of said conduit.
 5. In a double walled cryogenic storage vessel having a cold storage container wall separated from a warm outer wall by a vacuum space, the combination comprising: a low thermal conductivity conduit extending from said container wall through said vacuum space to said outer wall for carrying cold boil-off gas out of said storage container; a collar of highly thermally conductive material located in said vacuum space and surrounding a selected portion of the outer surface of said conduit so as to be in heat transfer relationship with said outer surface for at least about one inch along the length thereof; a highly thermally conductive shield affixed to said collar and extending into said vacuum space; and, restriction means in said conduit for restricting the cross sectional area of said conduit along at least the same length as that for which said collar extends, said restriction means being operative to increase the velocity of said boil-off gas as it passes through said selected portion.
 6. The apparatus according to claim 5 wherein the restriction means is comprised of a packing material in said conduit in heat transfer relationship with the portion of the inner surface of said conduit adjacent said selected portion of said outer surface.
 7. The apparatus of claim 6 wherein said restriction means has an effective thermal conductivity of about that of terephthalic ester polymer fibers.
 8. The apparatus of claim 6 wherein said packing material is fibrous and comprised of fibers having diameters of less than about 100 microns.
 9. The apparatus of claim 8 wherein said fibers are comprised of terephthalic polymer fibers.
 10. The apparatus according to claim 5 including a second low thermal conductivity conduit located inside of the first conduit to form a channel for the flow of said boil-off gas between the inner wall of said first conduit and the outer wall of said second conduit, said restriction means being located in said channel; and, means for normally preventing the flow of boil-off gas through said second conduit.
 11. The apparatus of claim 10 wherein said means for normally preventing the flow of boil-off gas through said second conduit is responsive to more than a predetermined amount of pressure in said storage container to permit said boil-off gas to pass through said second conduit so as to relieve said pressure.
 12. The apparatus of claim 10 wherein the restriction is comprised of a packing material in heat transfer relationship with the outside surface of said second conduit and the inSide surface of said first conduit.
 13. The apparatus of claim 10 wherein said restriction means has an effective thermal conductivity of about that of terephthalic ester polymer fibers.
 14. The apparatus of claim 10 wherein said packing material is fibrous and comprised of fibers having diameters of less than about 100 microns.
 15. The apparatus of claim 14 wherein said fibers are comprised of terephthalic ester polymer fibers.
 16. The apparatus of claim 10 wherein said second conduit is spaced from said first conduit by no more than about one-eighth inch.
 17. In a double walled cryogenic storage vessel having a cold storage container wall separated from a warm outer wall by a vacuum space, the combination comprising: a low thermal conductivity conduit extending from said container wall through said vacuum space to said outer wall for carrying cold boil-off gas out of said storage container; a collar of highly thermally conductive material located in said vacuum space and surrounding a selected portion of the outer surface of said conduit so as to be in heat transfer relationship with said outer surface for at least about one inch along the length thereof; a highly thermally conductive shield affixed to said collar and extending into said vacuum space; and, a fibrous packing material in said conduit in heat transfer relationship with the portion of the inner surface of said conduit adjacent said selected portion of said outer surface.
 18. The apparatus of claim 17 wherein said packing material has an effective thermal conductivity of about that of terephthalic ester polymer fibers.
 19. The apparatus of claim 18 wherein said fibers have a diameter of less than about 100 microns.
 20. The apparatus of claim 17 wherein said packing material is comprised of terephthalic ester polymer fibers having diameters of less than about 100 microns.
 21. The apparatus of claim 17 including a second low thermal conductivity conduit located inside of the first conduit to form a channel for the flow of said boil-off gas between the inner wall of said first conduit and the outer wall of said second conduit, said fibrous packing material being located in said channel; and, means for normally preventing the flow of boil-off gas through said second conduit.
 22. The apparatus of claim 21 wherein said second conduit is spaced from said first conduit by no more than about one-eighth inch.
 23. The apparatus of claim 21 wherein said means for normally preventing the flow of boil-off gas through said second conduit is responsive to more than a predetermined amount of pressure in said storage container to permit said boil-off gas to pass through said second conduit so as to relieve said pressure.
 24. The apparatus of claim 21 wherein said packing material has an effective thermal conductivity of about that of terephthalic ester polymer fibers.
 25. The apparatus of claim 24 wherein said fibers have diameter of less than about 100 microns.
 26. The apparatus of claim 21 wherein said packing material is comprised of terephthalic ester polymer fibers having diameters of less than about 100 microns.
 27. The apparatus of claim 17 including at least a second highly thermally conductive collar located in said vacuum space and surrounding at least a second selected portion of the outer surface of said conduit so as to be in heat transfer relationship with said outer surface, the height of the collar closer to said cold storage container wall being less than the height of a collar being further from said storage container wall.
 28. The apparatus of claim 27 including a highly thermally conductive shield affixed to each of said collars, each of said shields extending into said vacuum space.
 29. The apparatus of claim 27 wherein said collar closer to said cold storage container is at least about one inch high.
 30. The apparatus of claim 27 wherein said packing material has an effective thermal conductivity of about that of terephthalic ester polymer fibers.
 31. The apparatus of claim 30 wherein said fibers have a diameter of less than about 100 microns.
 32. The apparatus of claim 27 wherein said packing material is comprised of terephthalic ester polymer fibers having diameters of less than about 100 microns. 